Retinoic acids function through binding to a family of proteins known as the nuclear retinoic acid receptors ("RAR"). Three members of this family are designated RAR.alpha., RAR.beta., and RAR.gamma.. 9-cis retinoic acid binds to a group of receptors known as the retinoic acid X receptors ("RXR") (Levin et al., Nature, 355:359-361 (1992); Heyman et al., Cell, 68:397-406 (1992)), while all-trans- and 13-cis-retinoic acid, which bind to RAR.alpha., do not. However, 9-cis retinoic acid also acts as a ligand for the RAR group of receptors, and therefore may display the undesirable side effects of all-trans and 13-cis retinoic acid. Both RAR and RXR receptors are known to form heterodimers to mediate their biological functions (Leid et al., Cell, 68:377-395 (1992); Yu et al., Cell, 67:1251-1266 (1991); Zhang et al., Nature, 355:441-446 (1992).